As discussed in patent literature 1, inventors of the present invention have proposed a method and an apparatus for switching an optical path based on a new principle, according to which a control light absorbing region of a thermal lens forming element is irradiated with control light having a wavelength selected from a wavelength band absorbed by the control light absorbing region and signal light having a wavelength selected from a wavelength band not absorbed by the control light absorbing region. When the control light and the signal light are converged, their optical axes accord with each other. According to the above-described method and apparatus, the optical path changes in the following manner. If the thermal lens forming element is not irradiated with the control light, the signal light travels straight via a hole of a mirror. On the other hand, if the thermal lens forming element is irradiated with the control light, the signal light is reflected by the holed mirror which is inclined relative to the traveling direction of the signal light. Background arts of the invention are described in detail in patent literature 1.
As discussed in patent literature 2, the inventors of the present invention have also proposed an optical control system optical path switching type light signal transmission apparatus and a method for switching light signal optical path, according to which usage of a plurality of thermal lens forming elements combined with holed mirrors is described. In the optical path switching system discussed in patent literature 1 and patent literature 2, when the thermal lens forming element is irradiated with the control light, the signal light becomes a beam having a ring shape in its cross section due to thermal lens effect. Hence, this system is referred to as “ring beam system.”
Furthermore, as discussed in patent literatures 3 to 5, the inventors of the present invention have proposed a light deflection method and an optical path switching apparatus, according to which a thermal lens forming optical element has a control light absorbing region, wherein control light having a wavelength selected from a wavelength band absorbed by the control light absorbing region and signal light having a wavelength selected from a wavelength band not absorbed by the control light absorbing region are incident on the control light absorbing region of the thermal lens forming optical element. The control light absorbing region is convergently irradiated with the control light and the signal light. The control light and the signal light have convergence points different in their positions. Both the control light and the signal light converge and then diffuse on or near an incident plane of the control light absorbing region in the light traveling direction. Accordingly, the temperature of the control light absorbing region increases at a portion where the control light is absorbed and its peripheral region, and a thermal lens is reversibly formed in this region. The refractive index varies and, as a result, the signal light changes its traveling direction.
In patent literatures 3 to 5, the control light absorbing region of a thermal lens forming optical element is configured as a glass container filled with a solution containing at least one dyestuff dissolved in a solvent. The solvent, capable of dissolving the dyestuff, is the one not thermally decomposed when the temperature rises in the process of thermal lens formation. It is desired that the boiling point of the solvent is not lower than 100° C., preferably not lower than 200° C., more preferably not lower than 300° C.
However, patent literatures 3 to 5 describe nothing about temperature dependency in refractive index and viscosity of the solvent. In the optical path deflection system discussed in patent literatures 3 to 5, the signal light maintains a circular shape in its beam cross section even when the thermal lens forming optical element is irradiated with the control light. Hence, this system is referred to as “circular beam system.”
As discussed in patent literature 6, some of the inventors of the present invention have proposed an optical control method, according to which an optical cell filled with a photosensitive liquid composition is irradiated with control light having a wavelength to which the photosensitive composition is sensitive. The optical control method includes reversibly changing the transmissivity and/or refractive index of signal light having a wavelength selected from a wavelength band different from that of the control light, to perform intensity modulation and/or light flux density modulation on the signal light passing through the optical cell.
The optical control method includes irradiating the optical cell with the control light and the signal light converged thereon. The optical paths of the control light and the signal light are disposed so that regions near the foci of the control light and the signal light, where the photon density is highest, are overlapped with each other in the photosensitive composition of the optical cell. The pencil of light of the signal light diffusing after passing through the photosensitive composition in the optical element is received by a convex lens or a concave mirror having a numerical aperture smaller than that of a converging unit of the signal light. Thus, the pencil of light of the signal light in a region strongly subjected to the intensity modulation and/or light flux density modulation can be separately taken out.
However, patent literature 6 describes nothing about temperature dependency in refractive index and viscosity of the solvent. A thermal lens forming element discussed in patent literature 6 includes two glass plates of an optical cell and a spacer, which constitute a flatten cuboidal space filled with a dyestuff solution. However, patent literature 6 describes nothing about variation in thermal lens effect occurring when the orientation of the element relative to the direction of gravity is changed.
In the history of laser optics, there are numerous researches and reports relating to the “thermal lens effect”, i.e., a phenomenon occurring when an optical medium is irradiated with a laser, wherein refractive index and its distribution are changed according to an increase in temperature caused by heat generation. For example, non-patent literature 1 discusses about ∂n/∂T (a temperature coefficient of variation in refractive index) observed when a total of twenty-seven types of organic solvents are irradiated with a helium-neon laser having an oscillation wavelength of 633 nm. However, non-patent literature 1 describes nothing about temperature dependency in viscosity of the solvent.    Patent literature 1: JP 3809908 B    Patent literature 2: JP 3906926 B    Patent literature 3: JP 2007-225825 A    Patent literature 4: JP 2007-225826 A    Patent literature 5: JP 2007-225827 A    Patent literature 6: JP 3504076 B    Non-patent literature 1: D. Solimini: “Loss Measurement of Organic Materials at 6328 Å”, J. Appl. Phys., vol. 37, 3314-3315 (1966)